Magnetic properties of spinel-type oxides NiMn2-xMexO4

New materials, based on the well-known spinel compound NiMn2O4, have been synthesized and characterized from the magnetic point of view. The manganese cation was partially substituted in the general formula NiMn2-xMexO4 , by nonmagnetic and magnetic elements, such as Me = Ga, Zn, Ni and Cr (0 x 1). Prior to the determination of their magnetic properties, the non-substituted spinel NiMn2O4 was carefully characterized and studied as a function of the oxygen stoichiometry, based on the influence of the annealing atmosphere and quenching rate. The ferrimagnetic character was observed in all samples, with a paramagnetic-to-ferromagnetic transition temperature Tc stabilized at 110 K, and well defined long-range antiferromagnetic interactions at lower temperatures, which depend on the applied field and the substitute concentrationAuthors from Chile and O.P. thank projects Fondecyt-Chile 1020066, 7020066 and 1050178. Authors from France and Brazil thank project CAPES/COFECUB 416/03. Authors from France thank Région Bretagne for financial supportPeer reviewe

Εξαρτώμενη από το σπιν ηλεκτρονική μεταφορά και μαγνητική απεικόνιση νανοκλίμακας δομών μετάλλου/πυριτίου

In this work, we experimentally study spin-dependent hot electron transport throughmetallic multilayers (ML), containing single magnetic layers or “spin-valve” (SV) trilayers.For this purpose, we have set up a ballistic electron emission microscope (BEEM),a three terminal extension of scanning tunnelling microscopy on metal/semiconductorstructures. The implementation of the BEEM requirements into the sample fabricationis described in detail. Using BEEM, the hot electron transmission through theML’s was systematically measured in the energy range 1-2 eV above the Fermi level.By varying the magnetic layer thickness, the spin-dependent hot electron attenuationlengths were deduced. For the materials studied (Co and NiFe), they were compared tocalculations and other determinations in the literature. For sub-monolayer thickness,a non uniform morphology was observed, with large transmission variations over subnanometricdistances. This effect is not yet fully understood. In the imaging mode,the magnetic configurations of SV’s were studied under field, focussing on 360◦ domainwalls in Co layers. The effects of the applied field intensity and direction on the DWstructure were studied. The results were compared quantitatively to micromagneticcalculations, with an excellent agreement. From this, it can be shown that the BEEMmagnetic resolution is better than 50 nm.Σε αυτή την εργασία, μελετήθηκε πειραματικά η μεταφορά θερμών ηλεκτρονίων που εξαρτάται από το σπιν, σεμεταλλικά πολυστρωματικά υμένια, που περιέχουν μεμονωμένα μαγνητικά στρώματα ή τριστρώματα τύπου «βαλβίδας σπιν».Για το σκοπό αυτό, δημιουργήθηκε ένα μικροσκόπιο βαλλιστικής εκπομπής ηλεκτρονίων. Οι απαιτήσεις για την υλοποίηση της εν λόγω τύπου μικροσκοπίας περιγράφονται αναλυτικά. Χρησιμοποιώντας το μικροσκόπιο, η μετάδοση θερμού ηλεκτρονίου μέσω των πολυστρωματικών υμενίων μελετήθηκαν συστηματικά στο ενεργειακό εύρος 1-2 eV πάνω από το επίπεδο Fermi.Μεταβάλλοντας το πάχος του μαγνητικού στρώματος, εξήχθησαν οι αποστάσεις εξασθένησης του ρεύματος σπιν. Οι παράμετροι για τα υπό μελέτη υλικά (Co και NiFe), συγκρίθηκαν με αυτές από τη βιβλιογραφία. Για πάχος υπομονοστοιβάδας, παρατηρήθηκε μη ομοιόμορφη μορφολογία, με μεγάλες διακυμάνσεις μετάδοσης έναντι της υπονανομετρικήςαπόστασης. Αυτή η επίδραση δεν είναι ακόμη πλήρως κατανοητή. Στη λειτουργία απεικόνισης,οι μαγνητικές δομές των πολυστρωματικών υμενίων μελετήθηκαν υπό πεδίο, εστιάζοντας στην περιοχή όπου εμφανίζονται τοίχοι 360◦σε στρώματα Co. Μελετήθηκαν οι επιπτώσεις της εφαρμοζόμενης έντασης και κατεύθυνσης μαγνητικού πεδίου στη δομή των τειχών. Τα αποτελέσματα συγκρίθηκαν ποσοτικά με μικρομαγνητικούς υπολογισμούς, με εξαιρετική συμφωνία. Αποδεικνύεται ότι η χωρική μαγνητική ανάλυση της μεθόδου είναι καλύτερη από 50 nm

Torsional resonance mode magnetic force microscopy: Enabling higher lateral resolution magnetic imaging without topography-related effects

We present experimental work that reveals the benefits of performing magnetic force microscopy measurements employing the torsional resonance mode of cantilever oscillation. This approach provides two clear advantages: the ability of performing magnetic imaging without topography-related interference and the significant lateral resolution improvement (approximately 15%). We believe that this work demonstrates a significant improvement to a versatile magnetic imaging technique widely used in academia and in industry. © 2013 IOP Publishing Ltd.AK received funding from the EC through a FP7 Intra-European Marie Curie post-doctoral Fellowship (Grant No. PIEF-GA-2010-272470). Funding from the Spanish MINECO (refs. MAT2011-29194-C02-01, MAT2010-10213-E and CSD2008-00023) and Comunidad de Madrid (S2009/MAT-1726) is also acknowledged.Peer Reviewe

Torsional Resonance mode Magnetic Force Microscopy

Comunicación presentada en el 2nd Early Stage Researchers Workshop in Nanoscience, celebrado en Madrid el 28 y 29 de junio de 2012.Torsional Resonance mode Atomic Force Microscopy (TR-AFM) has emerged during the last decade as a promising dynamic AFM method for imaging lateral forces. Traditional dynamic AFM modes of operation, e.g. Tapping Mode AFM (TM-AFM), employ flexural (>diving board>) cantilever oscillations. On the contrary, TR-AFM takes advantage of torsional (twisting) cantilever oscillations. In this work, TR mode Magnetic Force Microscopy (TR-MFM) measurements are presented. A double-pass method is used for performing MFM: a TM-AFM main scan yields the surface topography, while the long range magnetic forces are detected using a lifted TR-MFM scan (or a TM- MFM scan, for comparison). The main advantage of TR-MFM originates from the fact that the flexural cantilever oscillation is only excited thermally. As a consequence, the corresponding oscillation amplitude is around 1 nm, up to one order of magnitude lower than in the case of TM-MFM. This provides the ability of performing dynamic MFM measurements with significantly reduced tip-sample distance (the tip can even be lowered during the “lifted” scan). As a result, improved spatial resolution can be achieved (see figure), while preserving a high signal-to-noise ratio. Taking into account the above-mentioned advantages, it is argued that TR-MFM provides a significant improvement to a prolific magnetic imaging method.Peer Reviewe

Unraveling the magnetic domain structure of nanopatterned hard/soft bilayer antidot arrays with long-range periodicity

Trabajo presentado en el CMD 2020 GEFES, celebrado online del 31 de agosto al 4 de septiembre de 2020Magnetic antidot arrays are groups of ordered holes patterned on a continuous magnetic film, which are being studied for several applications such as magnetically-active plasmonic media, microwave devices, and magnetic sensors, to name a few. In this work [1], a top-down approach with focused ion beam has been used to fabricate Co/Permalloy hard/soft bilayer magnetic antidot arrays with square and hexagonal symmetries (antidot diameter: 40 nm; lattice constant: 240 or 360 nm). The long-range periodicity of these arrays results in higher magnetic coercivity and stronger magnetic domain-wall pinning, compared to identical hard/soft bilayers of short-range order antidots deposited on porous alumina [2]. Finally, Magnetic Force Microscopy imaging shows striking qualitative differences between the two symmetries: hexagonal arrays show a homogeneous magnetic configuration, whereas square ones have super-domains (regions with uniform magnetization texture) separated by super domain-walls (SDWs), see Figure 1. Two kinds of SDWs are observed: low stray field energy (LESDWs) that are linear and expand over several lattice constants, and high stray field energy (HE-SDWs) that occupy only a few lattice constants and are situated at kinks of LE-SDWs